1. Field of the Invention
The present invention relates to an implantable cardiac device of the type having a heart stimulator for electrically stimulating the heart of a patient, detecting means for measuring a physiologic parameter that is affected by the status of a cardiovascular disease associated with sympathetic activation, signal processing means for determining at least one of a low frequency, LF, and a very low frequency, VLF, Mayer wave component in the measured parameter, and an analyzer for analyzing the determined Mayer wave component in relation to a predetermined reference value to determine the status of the cardiovascular disease, and to a corresponding method for monitoring the status of a cardiovascular disease associated with sympathetic activation of a patient having an implantable electric heart stimulator.
2. Description of the Prior Art
Heart Rate Variability, HRV, has been suggested as a parameter reflecting the activity in the Autonomic Nervous System, ANS. This activity is altered with the health of a patient. The pathophysiologic activation of ANS at heart failure and other diseases reveal the level of stress and unbalance in the body. HRV diminishes during heart disease and can for instance be used to detect and show deterioration of a Heart Failure, HF, and predict sudden cardiac death. Unfortunately HRV can only be measured during sinus rhythm. For patients having their heart rate modulated to a large extent by an Implantable Cardiac Device, such as a pulse generator or ICD, calculation of HRV is not feasible.
Mayer waves are low frequency oscillations in ANS causing a. o. blood pressure variations and HRV. The phenomenon is not well understood but its existence and response to heart disease, and also other circumstances, have been confirmed, see e.g. Cardiovascular Research, 70, 2006, pp. 12-21, Circulation 95, 1997, pp. 1449-54, and J. Hypertens., 17(12 Pt 2), December 1999, pp. 1905-10.
EP 1 151 719 discloses an implantable apparatus for monitoring the condition of a heart failure patient using respiration patterns. The patient's respiratory patterns are monitored to identify periodic breathing or Cheyne-Stokes respiration. Different parameters are suggested for assessing Cheyne-Stokes respiration, like mechanical changes of thorax due to breathing, changes in blood and tissue pH, CO2 concentration and the R-R interval, extracted from ECGs. From R-R interval information measures of HRV are derived and the presence of respiratory fluctuations as well as Mayer waves are tested by examining the variability over specific frequency ranges. The absence of fluctuations in the respiratory fluctuation frequency band as well as in the Mayer waves frequency bands is interpreted as a worsening of the disease status.
In U.S. Pat. No. 5,645,570 an implantable device for detecting the sympatho-vagal balance of a patient is described. From ECGs the variability of the heart rate is evaluated as the number of consecutive R-R intervals, which differ from one another by at least a minimum threshold. If this number satisfies a predetermined intervention criterion a therapeutic device for the patient is triggered.
In Lee A. Fleisher, “Heart Rate Variability as an Assessment of Cardiovascular Status”, Journal of Cardiothoracic and Vascular Anesthesia, Vol. 10, No. 5 (August), 1996, pp. 659-671 the usage is described of time intervals between consecutive heart beats, preferably the R-R intervals, for evaluating HRV. The R-R intervals are derived from ECGs. The evaluation can be made in the time domain or in the frequency domain. In the latter case the power spectrum in the Mayer wave frequency range can be used to predict mortality of Congestive Heart Failure, CHF, patients.
The above discussed prior art for HRV analysis, e.g. the use of the R-R interval measurements for assessment of HRV, is applicable only to patients in sinus rhythm, cf. e.g. the above mentioned article Lee A. Fleisher, “Heart Rate Variability as an Assessment of Cardiovascular Status”, Journal of Cardiothoracic and Vascular Anesthesia, Vol. 10, No. 5 (August), 1996, p. 662, left column, third paragraph. For patients having their heart rate modulated or controlled by a heart stimulator it is, however, not possible to use the traditional HRV analysis.
It has, however, appeared that e.g. variability of blood pressure and blood flow are possible markers for studying pathophysiologic activation of ANS at heart failure and other diseases. The strength of the arterial blood pressure oscillations, for instance, is highly affected at some cardiovascular diseases associated with sympathetic activation, e.g. congestive heart failure, CHF. There are also physiologic parameters other than the blood pressure that are affected during sympathetic activation. Thus the left ventricular tension and the left ventricular contraction pattern are also indicators of sympathetic activation as well as the mechanical AR-interval and pre-ejection time.